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Get weekday from a date, C version
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| /** | |
| * Weekday calculation utils. | |
| * | |
| * Month and weekday are 1-based, 1 is Monday. | |
| * The enums may be replaced by ints at the expense of clarity. | |
| * | |
| * Released to the public domain | |
| */ | |
| #include <stdint.h> | |
| #include <stdbool.h> | |
| // Start year for weekday computation | |
| // - reduces loop iteration count if we know older years won't be needed | |
| #define DT_START_YEAR 2019 | |
| // Weekday on January 1st of DT_START_YEAR | |
| #define DT_START_WKDAY TUESDAY | |
| enum weekday { | |
| MONDAY = 1, | |
| TUESDAY, | |
| WEDNESDAY, | |
| THURSDAY, | |
| FRIDAY, | |
| SATURDAY, | |
| SUNDAY | |
| }; | |
| enum month { | |
| JANUARY = 1, | |
| FEBRUARY, | |
| MARCH, | |
| APRIL, | |
| MAY, | |
| JUNE, | |
| JULY, | |
| AUGUST, | |
| SEPTEMBER, | |
| OCTOBER, | |
| NOVEMBER, | |
| DECEMBER | |
| }; | |
| /** Days from Jan 1st to the 1st of a month */ | |
| static const uint16_t MONTH_YEARDAYS[] = { | |
| [JANUARY]=0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334 | |
| }; | |
| /** Check if a year is leap */ | |
| static inline bool is_leap_year(int year) | |
| { | |
| return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0); | |
| } | |
| /** Get weekday */ | |
| enum weekday date_weekday(uint16_t year, enum month month, uint8_t day) | |
| { | |
| uint16_t i; | |
| uint16_t days = (DT_START_WKDAY - MONDAY) + (year - DT_START_YEAR) * 365 + MONTH_YEARDAYS[month] + (day - 1); | |
| for (i = DT_START_YEAR; i <= year; i++) { | |
| if (is_leap_year(i) && (i < year || month > FEBRUARY)) days++; | |
| } | |
| return MONDAY + days % 7; | |
| } |
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